Electrical circuit arrangements for converting a variable rate of pulse transmission into a related electrical output quantity

ABSTRACT

An anti-lock vehicle brake control system comprising a wheel sensor for generating a train of pulses at a frequency proportional to the wheel speed. A switching transistor receives the pulses. A first capacitor is coupled to the output of the switching transistor via a first isolation diode. The switching transistor causes the first capacitor to charge up to a predetermined voltage level for each pulse received. A discharge circuit allows the first capacitor to discharge in the interval between successive pulses. A second capacitor is permanently connected to a source of charging current and is also coupled to the first capacitor via a second isolation diode so that the second capacitor will discharge towards the voltage level on the first capacitor to produce a voltage on the second capacitor that is proportional to wheel speed. The latter voltage is used to actuate a control valve in the vehicle brake fluid pressure system to relieve the braking pressure whenever said capacitor voltage exceeds a given value.

United States Patent 51 3,661,428

Sharp 1 May 9, 1972 [s41 ELECTRICAL'CIRCUIT 53,245,213 4/1966 Thompson m1. .303/21 EM NT -F C NVE TI A 3,482,887 12/1969 Sheppard ..303/21B VARIABLE RATE OF PULSE 1 OTHER PUBLICATIONS TRANSMISSION INTO A RELATED ELECTRICAL OUTPUT QUANTITY Denis Sharp, C rawley, Sussex, England u.s. Philips Corporation, New York, NY. Sept. 16, 19 9' Assignee:

Filed:

Appl. No.:

US. Cl. ..303/2l CF,.188/l81 A, 303/21 CG, 307/233, 307/295, 320/1 Int. Cl. ..B60t 8/08, B60t 8/12 FieldoiSearch ..188/l81,l81A, l8lC; 303/21, 303/20; 320/1; 307/233, 295; 324/160, 161, 162;

Reierences Cited UNITED STATES PATENTS 10/1962 Buttenhofi 4/1966 Anderson et al.

7/1966 Willard ..307/233 X 5/1970 Fielek,.lr. .....303/2l EB 7/1970 French at al. ..303/2l BE 303/21 BE UX .....303/2l EB May et al., British Patent Specification, Ser. No. 880,767, filed Primary Examiner -Milton Buchler Assistant Examiner-Stephen G. Kunin Attorney-Frank R. Trifari ABSTRACT An anti-lock vehicle brake control system comprising a wheel sensor for generating a train of pulses at a frequency proportional to the wheel speed. A switching transistor receives the pulses. A first capacitor is coupled to the output of the switching transistor via a first isolation diode. The switching transistor causes the first capacitor to charge up to a predetermined voltage level for each pulse received. A discharge circuit allows the f rst capacitor to discharge in the interval between successive pulses. A second capacitor is permanently connected to a source of charging current and is also coupled -to the first capacitor via a second isolation diode so that the second capacitor will discharge towards the voltage level on the first capacitor to produce a voltage on the second capacitor that is proportional to wheel speed. The latter voltage is used to actuate a control valve in the vehicle brake fluid pressure system to relieve the braking pressure whenever said capacitor voltage exceeds a given value.

7.Clairns, 5 Drawing Figures Allll AAAAA Allll PATENTEDMM 9 I972 3,661,428

SHEET 1 0F 3 INVENTOR.

DE N I S 8 HA R P AG NT PATENTEDHAY 9 1912 SHEET 2 [1F 3 P'CKUP AMPL- CONVERTER DIFE ANTILOCK SERVO SOLENOID ail Fig.3]

INVENTOR.

0 E N l s 5 HA R P PATENTEDMAY 9 I972 7 3,661,428

SW11 3 OF 3 CCM SE W common. SENSOR cmcun ANTILOCK MASTER V CYLINDER OL BRIEKE WB CU FP MC WHEEL To OTHER BRAKE UNITS Fig.5

INVENTOR.

0 E N s SHARP BY M x. AGEN second capacitor in accordance with the level This invention relates to electrical circuit arrangements for converting a variable rate of pulse-transmission'into a related electrical output quantity, and niorespecifically to such circuit arrangements of a kind having a firstcapacitor arranged to be charged in response toeach pulse of a train of pulses applied to thearrangement, the charge on this first capacitor decaying between successive pulses, and'a second capacitor arranged to assume a level of charge in accordance with the level ofv the charge on the first capacitor, said-output quantity being derived from the second capacitor. A'circuit arrangement of this kind is disclosed in U.S. Pat. No. 3,508,074 Specification No. 1,143,092 (PI-IB 31561).

According to the present invention an electrical circuit arrangement of the above kind comprises, transistor switching means responsive to each pulse of a train capacitor to be charged to a predetermined level through a first isolating diode, the charge on said first capacitor then decaying until the beginning of the next pulse, a charging circuit in which said second capacitor is permanently connected, and a second isolating diode through which the charge on said second capacitor can decay to or towardsthe level of the charge on said first capacitor. The output quantity is derived from said of charge thereon.

In the operation of an'electrical circuit arrangement according to the invention,-the frequency at which the first capacitor is repetitively charged to said predetermined level corresponds to the instantaneous frequency'of the applied train of pulses so that the level of charge remaining on the first capacitor immediately before it is recharged to said predetermined level on the occurance of each pulse increases with an increase in the pulse frequency; and'vice-versa.

The second capacitor is continually attempting to charge up over its permanent charging circuit, but its charge is also decaying through the second isolating diode to or towards the level of the charge remaining on the. first capacitor, so that the resultant level of the charge on the second capacitor is determined by the level of the charge to which the first capacitor is decaying,'which level, as aforesaid, isdetermined by the pulse frequency. The output quantity produced by the arrangement is preferably the voltage existing across the second capacitor as determined by the level of charge thereon.

It is .envisaged that an electrical circuitarrangement accord ing to the invention has a particular application in so-called anti-lock brake systems for wheeled vehicles, that is, systems designed to improve braking performance of a vehicle by relieving the braking pressure applied-to a wheel if the wheel tends to lock on a slippery surface following brake application and then increasing the braking pressure again without the need for any change in the actual braking action causing the brake application. Such systems can be successful in reducing the risk of skidding due to wheel lock and in maintaining directional control and can also reduce braking distances.

This envisaged application of the electrical circuit arrangement is in control circuit means of an anti-lock vehicle brake system of the character comprising, for use in combination with a vehicle wheel and associated wheel brake, a wheel sensor for producing electrical signals related to rotational movement of the wheel, control circuit means which is responsive to said electrical signals to produce an electrical output in dependence on a particular criterion related to wheel rotational movement, and control valve means which is arranged for actuation in response to said electrical output to cause braking pressure as applied from a fluid pressure source of the system to the wheel brake to be relieved. A suitable criterion though not the only one is when the deceleration of the wheel is in excess of a predetennined value.

In this application, the electrical circuit arrangement can provide a voltage of value related to the frequency of a pulse train (constituting said electrical signals) which is generated in respo'nseto wheel rotational movement, the'frequency of the pulse train being related to wheelspeed. Typically, the pulse train'may be produced by magnetic interaction between a ferromagnetic toothed ring attached to the wheel and an electromagnetic pick-up which is positioned adjacent to the ring to sense the change of fiux as each tooth of the ring passes'it and is succeeded by a gap when the wheelrevolves, said ring and pick-up constituting the wheel sensor. The resulting voltage output, which is related to thefrequency of the pulse train, can be utilised in the control circuit means to detennine when the electrical output is to be produced to cause the actuation of the control valve means. The latter is suitably a solenoid or other electro-magnetic device responsive to said electrical output to produce mechanical movement for actuating an associated valve. I

The present invention also provides an anti-lock vehicle brake system of the above character having control circuit means embodying an electrical circuit arrangement as set forth above.

In order that the invention maybe more fully understood reference will now be made by way of example to the accompanying drawings, in which:

FIGS. 1 and 2 show respective embodiments of an electrical circuit arrangement conforming to the invention;

FIG. 3 is a block diagram of a control circuit means of an anti-lock vehicle brake system of the character referred to;

FIG. 4 is a circuit diagram of the control circuit means of FIG. 3; and

FIG. 5 is a block diagram of an anti-lock vehicle brake system of the character referred to.

Referring to the drawings, the electrical circuit arrangement shown in FIG. 1 comprises an input transistor T1 having its base connected to one end of an output coil L of a pick-up device (not otherwise shown) which is arranged to produce a train of pulses forapplication to the base of transistor T1. The other end of the coil L is connected to the junction of two resistors R1 and R2 which are connected in series between an earth line E and a positive voltage line VI.

The collector of the transistor T1 is connected to the positive voltage line V1 via two series connected resistors R3 and R4 and its emitter is connected directly to the earth line E. A switching transistor T2 has its base connected on the one hand to the junction of the resistors R3 and R4 via a capacitor C1 and on the other hand to the positive voltage line V1 via a resistor R5. The emitter of the transistor T2 is connected directly to the earth line E and its collector is connected to the positive voltage line Vl via a resistor R6. There is connected to the collector of transistor T2 one end of the parallel combination of a resistor R7 and a diode D1. The other end of this parallel combination is connected to one side of a capacitor C2, the other side of which is connected to the earth line E. A capacitor C3 is connected in series with a resistor R8 between the earth line E and a positive voltage line V2. The junction of this capacitor C3 and resistor R8 has an output terminal OT connected to it, and this junction isalso connected via a diode D2 to the common junction of resistor R7, diode D1 and capacitor C2.

When the electrical circuit arrangement of FIG. I is energized by the application of suitable supply voltages across the earth line E and the positive voltage lines V1 and V2, the switching transistor T2 is rendered conductive, while the input transistor T1 is initially biased at the threshold of conduction. Upon the application of a train of pulses from the pick-up device output coil L to the base of transistor T1 this transistor is rendered conductive in response to each pulse of the train to effect amplification and limiting at .the pulse train frequency. The resulting output at the collector of transistor T1 is a square wave voltage which is applied to the capacitor C1. The capacitor C1, in conjunction with resistor R5 differentiates each cycle of this square wave voltage. Thus, there is applied to the base of the switching transistor T2 negative-going voltage spikes which render this latter transistor non-conductive fora short periodonce on the capacitor C2 is decaying through resistor R7 and transistor T2 which is now conductive again, so that the voltage across the. capacitor C2 decreases. The extent of this decrease depends upon the time interval between successive negative-going voltage spikes applied to the base of transistor T2 and thus upon the instantaneous frequency of the applied train of pulses.

1 Since the capacitor C3 is permanently connected in series with resistor R8 between the-earth line E and the positive voltage line V2, this capacitor will immediately begin to charge up as soon as the circuit arrangement is energized by the application of suitable supply voltages. However, as soon as the voltage across the capacitor C3 exceeds the voltage across the capacitor C2, the diode D2 becomesforward biased so that the charge on capacitor C3 decays through diode D2 to or towards the level of the chargeon the capacitor C2. Therefore, the value of the voltageacross the capacitor C3 tends to follow the value of the voltage across the capacitor C2 as the charge on the latter is decaying and the value to which the ,voltage across capacitor C2 has decayed is stored on the capacitor C3 each time the voltage across the capacitor is reset at the beginning of each applied pulse. If thefrequency of the applied train of pulses increases, then the voltage across -capacitor'C3 increases to a higher value which the voltage across capacitor C2 only has time to decay to, before resetting, because of the shorter time interval between successive pulses. Conversely, if the frequency of the applied train of pulses decreases, then the voltage across capacitor C2 decreases to a lower value because there is a longer time interval before this voltage is reset and the voltage across capacitor 4 Transistor TR2type Be 108 Mullard Resistor R2-270'ohms Capacitor Cl0.022uF Mullard Resistor R3- 10K ohms Capacitor C2-0. lpF Mullard Resistor R4- 10K ohms Capacitor C3- 1 .OpF Mullard Resistor R5 -.-56K ohms Capacitor C4 1 .OpF Mullard Resistor R6- 1 K ohms Diode Dl-type OA202 Mullard Resistor RT- 1 5K ohms Diode D2-type OA202 Mullard Resistor R8270K ohms Diode D3-type OA202 Mullard Resistor R9 l K ohms Resistor RIO-330 ohms Voltage V1'+9.l volts Voltage V2+l 2 volts l Turning now to FIG. 3, the control circuit means represented by the block diagram there shown is responsive to pulses related to rotational movement of a vehicle wheel. As aforesaid, these pulses may be produced by an electro-magnetic pick-up 1 which is associated with a ferromagnetic toothed ring attached to the wheel to sense the change of flux as each tooth of the ring passes it and is succeeded by a gap as the wheel revolves. The pulse output from the pick-up 1 is amplified by an amplifier 2 and then applied to a frequency-to- (stabilized) D.C, converter 3 which would be comprised by an electrical C3 is pulled down to this lower level. The voltage across capacitor C3 is the output voltage appearing at the output terminal OT of the arrangement.

=The electrical circuit arrangement shown in FIG. 2 is similar in many respects to the arrangement of FIG. 1 and, for the sake of convenience, corresponding components in these two arrangements have beengiven the same references. The main difference between the two arrangements is that in FIG. 2

there is, now only a single transistor TI. This transistor T1 is biased so that it undergoes a change of conductive state in response to each pulse of a train of pulses applied to the arrangement. Each time the transistor T1 changes its conductive state a'voltage pulse appears at its collector and this pulse is differentiated by capacitor C4 and resistor R9 to provide a positive going voltage spike, either at its leading edge or its trailing edge depending on its polarity which is applied through diode D1 to charge the capacitor C2. In this way the voltage across the capacitor C2 is effectively reset to the voltage of the positive voltage line V1, as in the arrangement of FIG. 1. The charge on the capacitor C2 thereafter commences to decay through resistor R7 until the voltage across capacitor C2 is reset again and the operation of the circuit arrangement is now in other respects as described for the arrangement of FIG. 1. The diode D3 in the arrangement of FIG. 2 serves to divert to the earth line ,5 the negative-going voltage spike arising from each differentiation. Without this diode D3, the capacitor C4 would tend to the charge which it acquired due to this negative going voltage spike.

FIGS. 1 and 2, the voltage of the positive voltage line +Vl is which is passed to a power amplifier 5,

circuit arrangement conforming to the invention and is effective to produce anoutput voltage of magnitude related to the frequency of the pulses supplied by the pick-up 1. This output voltage is differentiated by a differentiator 4, the output from and the output from the power amplifier 5 is utilized to operate a solenoid 6 adapted to actuate control valve means antilock servo 7).

In the circuit diagram of the control circuit means shown in FIG. 4, the pick-up is again represented only by its output coil L as in FIGS. 1 and 2. The pulse output from this pick-up output coil L is coupled into the base of a transistor Ta, which comprises the amplifier 2 in FIG. 3, via a capacitor Ca. A capacitor Cb serves to remove unwanted interference in the output from the output coil L, and a diode Da'serves to 'prevent the DC. bias at the base of transistor Ta, as provided square wave voltage which is coupled" into the base of a transistor Tb via a capacitor Cc. This capacitor Cc and a base resistor Rb for transistor Tb have values chosen such that the transistor Tb, being normally conductive, is rendered nonconductive to produce a positive pulse of fixed length atits collector for each cycle of square wave voltage coupled into its base. Each such positive pulse charges up a capacitor Cd through a diode Db to the stabilized voltage on the line SL. This stabilized voltage is provided by a Zener diode Zd which is connected in series with a resistor Rc across the voltage supply lines V and 0V. At the termination of each positive pulse at the collector of transistor Tb, capacitor Cd commences to discharge exponentially through a resistor Rd and transistor Tb. When the voltage across the capacitor Cd becomes negative with respect to the voltage across a capacitor Ce, a diode Dc becomes forward biased so that capacitor Ce also commences to discharge through the diode Dc, but at a much lower rate because its discharge time constant is much longer than the discharging time constant of capacitor Cd. However, each time capacitor Cd is'being re-charged, diode Dc is back biased, thus allowing capacitor Ce to charge up via a resistor Re with which it is connected in series across the voltage supply lines +V and 0V. The components Tb, Db, Rd, I Dc, Cd, Ce and Re essentially comprise an electrical circuit arrangement conforming to the invention and forming the stabilized to ensure accurate resetting of the voltage across the capacitor C2. This stabilized voltage may be provided, for example, by means of a Zener diode as shown in the circuit diagram ofFIG. 4. v

Suitable types and values for the components of theelectrical circuit arrangements of FIGS. 1 and 2 are as follows: Transistor TRl-type Be I08 Mullard Resistor R13I ohms frequency-to-D.C. converter 3 of FIG. 3. This-arrangement produces across capacitor Ce an output voltage whose value is related to the input frequency of the pulse output supplied by the pick-up, and may thus be termed a speed signal as it is directly related to wheel speed. This output voltage (speed signal) across capacitor Ce is coupled to the base of a normally conductive transistor Tc via a capacitor Cf and a resistor Rf. The value of this .capacitorCf and the value of a resistor Kg to which the capacitor is also coupled, determine a selected wheel deceleration at which transistor Tc and a further normally conductive transistor Td are rendered nonconductive in response to the value of speed signal then obtaining, to cause a normally non-conductive transistor Te to become conductive. The components Cf, Tc and'Td essentially comprise the differentiator 4 of FIG. 3. The resistor Rg, which together with resistor Rf forms a potential divider in the base circuit of transistor Tc, provides a current sufficient to drive the base of transistor Tc with about times the current needed to maintain the two transistors To and Td normally conductive. Thus the selected wheel deceleration at which transistor Te becomes conductive is virtually independent of the gains of the transistors Tc and Td. A resistor Rh in the collector circuit of transistor Tc serves to limit the base current of transistor Td, and a capacitor Cg and the resistor Rf in the base circuit of transistor Tc makes the circuit insensitive to ripple in the speed signal. A diode Dd serves to stabilize the base'current of the transistor Tc against temperature changes. A capacitor Chserves to prevent spurious oscillation at high frequencies since the transistors are capable of working up to 80 M/cs.

' The transistor Tf and a further transistor Tg amplify the output from transistor Te. These transistors Te, Tf and T3 form the power amplifier 5 of FIG. 3. The output from transistor Tg drives a solenoid S which corresponds to the solenoid 6 in FIG. 3. A diode De serves to clip overshoot voltage on the solenoid S when it is switched off, thereby preventing too high a voltage from being applied to the collector of transistor Tg.

The circuit parameters would be so chosen that the solenoid would be turned off when the wheel being sensed has accelerated up to the speed it would have been doing if it had continued to decelerate from its initial speed, at the instant of braking, at a rate equal to the selected wheel deceleration at which the solenoid was turned on.

It is also arranged that the solenoid S is turned off after a predetermined period, even if the wheel does not re-accelerate after the solenoid S has been turned on. This is achieved by means of capacitor Cf which in conjunction with resistor R3 serves as an A.C. coupling to differentiate the speed signal, so that after a certain period of energization of the solenoid, as determined by the time constant of this A.C. coupling, the transistors Tc and Td are rendered conductive again to render transistor Tg non-conductive to de-energize the solenoid. However, since the capacitor Cf and resistor R3 also determine the selected wheel deceleration, the time constant of the A.C. coupling afforded by these components cannot be varied, to vary the period before the solenoid is deenergized in the absence of wheel re-acceleration, without also varying the selected wheel deceleration. A separate A.C. coupling which is independant of capacitor Cf and resistor Rg suitably comprises a further capacitor connected in the base circuit of transistor Te, together with a further resistor connected between this base and the 0V line.

The circuit diagram of FIG. 4 may be modified in that if a capacitor Cf of larger value and higher gain transistors are used, the transistor Tc and its collector resistor Rh can be dispensed with and the junction of resistor Rf and capacitor Cg can then be connected directly to the base of transistor Td.

Also, in each of the circuits of FIGS. 1, 2 and 4 transistors of opposite type to those shown may be used with suitable adjustment of the voltage supply lines.

Suitable components and component values for the circuit diagram of FIG. 4 are as follows:

Resistors Ra- 1M ohms Rj-56K ohms Rb-3K ohms Rk-lK ohms Ila-150 ohms Rl- 10K ohms Rd- K ohms Rm-33K ohms Rz- 150K ohms lbw-4K ohms Rf--33K ohms Ral0K ohms Rg-470K ohms Rp 10K ohms Rib-470K ohms RqlK ohms Ri-l8K ohms Rr-ISO ohms Capacitors Transistors Ca.22p.F Ta-type BC 108 (Mullard) Cb-0. lpF Tb Cc.022p.F Tc Cd-0. l F Td Ce--l.O .F Te Cfl .OuF Tf-BFYSZ Cg-0. luF Tg-BDY l 0 Ch--2kpF Diodes Voltages Zd8.2v zener (Mullard) +V-l2 volts Da-type OA202 t. D De-BYZIO f FIG. 5 shows diagrammatically a general layout for an antilock vehicle brake system in which the present invention can be embodied. This layout shows a brake foot pedal F P for actuating the piston of a master cylinder MC which constitutes a fluid pressure source of the system. The master cylinder is arranged to actuate (directly or via a servo) a wheel brake WB for a vehicle wheel W via an anti-lock control unit CU. A wheel sensor SE applies electrical pulses related to wheel rotational movement to a control circuit means CCM. The antilock control unit CU would include control valve means which is arranged for actuation in response to an electrical output from the control circuit means CCM to cause braking pressure applied to the wheel brake WB to be relieved. This system is of the character previously referred to, and in the present in stance in which the control circuit means is in accordance with FIGS. 3 and 4, the electrical output would be produced from the control circuit means CCM when the deceleration of the wheel is in excess of a predetermined value. The wheel sensor WE would be the pick-up 1 and the solenoid 6 and antilock servo 7 (i.e. the control valve means) would be included in the anti-lock control unit CU.

AS indicated by the lead LL, separate systems as shown in FIG. 5 (with a common fluid pressure source) may be provided in respect of each road wheel of a vehicle, but it would also be possible to provide a single system for two (rear) wheels driven by a vehicle propellor shaft with a sensor associated with the shaft for producing the electrical signals related to wheel rotational movement. As an alternative, a single anti-lock control unit including control valve means may be provided in common for all the road wheels of a vehicle. In this case each road wheel would have its own wheel sensor and associated control circuit means and any of the latter would provide an electrical output to actuate the control valve means when the appertaining wheel tends towards a locked condition.

We claim:

I. An anti-lock vehicle brake control system comprising, a

source of fluid pressure to control the braking force on a vehicle wheel, a wheel sensor for producing a train of pulses at a frequency that is related to the vehicle wheel speed, transistor switching means coupled to said wheel sensor to receive said pulse train, a first isolating diode, a first capacitor coupled to said transistor switching means via said first diode, said transistor switching means being responsive to the pulses in said pulse train to cause said first capacitor to be charged to a predetermined voltage level via said first diode, circuit means for discharging said first capacitor during the period between successive pulses in the pulse train, a second capacitor, a charge circuit including a source of voltage permanently connected to said second capacitor, a second isolating diode coupling said first and second capacitors together so that the charge on the second capacitor can decay towards the level of charge on the first capacitor to produce a voltage on the second capacitor that is determined by the wheel speed, and control valve means actuated in response to a given voltage level on said second capacitor to relieve the fluid pressure applied to the vehicle brakes.

2. A brake control system as claimed in claim 1 wherein said transistor switching means comprises, an input transistor coupled to the wheel sensor, means for biasing said input transistor at its threshold of conduction so that each pulse received turns the transistor on thereby producing at its output electrode a square wave voltage at the pulse frequency, a differentiating circuit coupled to said transistor output electrode for differentiating each cycle of said square wave voltage, a switching transistor which is normally biased into conduction, and means coupling the output electrode of the switching transistor to the first diode and the input of said switching transistor to the output of the differentiating circuit so that the switching transistor is cut-off in response to the leading voltage spike produced therein during each cycle of the square wave voltage.

3. A brake control system as claimed in claim 2 wherein said switching transistor output electrode is the collector electrode, means connecting said first diode and said first capacitor in series to said collector electrode, a resistor connected in shunt with the first diode, a second resistor connecting said collector to a source of DC voltage, said first capacitor being charged via said second resistor and said first diode when said switching transistor is cut-off.

4. A brake control system as claimed in claim 3 wherein said circuit means for discharging the first capacitor comprises said shunt resistor and the emitter-collector path of the switching transistor.

5. A brake control system as claimed in claim 2 wherein said first and second diodes are series connected with opposite polarity between the switching transistor output electrode and the second capacitor.

6. A brake control system as claimed in claim 1 wherein said transistor switching means comprises, an input transistor coupled to the wheel sensor, means for biasing said input transistor at its threshold of conduction so that the transistor undergoes a change of its conductive state in response to each pulse received, a differentiating circuit coupled to said transistor output electrode for differentiating the output pulses appearing thereat to produce positive going and negative going voltage spikes, the voltage spikes of one polarity being applied through said first diode to charge said first capacitor, and a resistor connected in shunt with said first capacitor to provide a discharge path therefor.

7. A brake control system as claimed in claim 6 further comprising a third diode connected between the output of the differentiating circuit and ground and poled so as to bypass to ground the voltage spikes of the other polarity.

g;;g; UNITED STATES "PA' IENT OFFICE I DCERTIFICATE OF CORRECTION Patent No. 3,661,428 Dated y 9 2 Inventor(s) Denis Sharp It is certified that: error appears in the above-identified patent and that said Letters Patent are hereby cdrrected as shown below:

col 1, line 20, after "train" insert ofpulses applied to the arrangement to cause said first C01. 1, line 17, cancel "Specification No.. 1,143,092

(PHB 31561)",

Signed and sealed this 2mm dayml91a (SEAL) Attest:

ROBERT GOTTSCHALK EDWARD M.FLETCHER,JR. Attssting Officer Commissioner of Patents UNITED STATES PATENT OFFECE PO-IOSO (5/ g) 6 CETIFICATE 0F mwoN Patent No. 3,661,428 I Dated y 1972 Inventor(s) Denis Sharp It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

col. 1, line 20, after "train" insert of pulses applied to the arrangement to cause said first 7 Col 1 line 17 cancel "Specification N0 l ,143,092

'(PHB 31561)",

Signed and sealed this 31 35 day Qgflobgr 1912 a (SEAL) Attest:

EDWARD MuFI-IETCI'IER,JR ROERT GOTTSCHALK Commissioner of Patents Attesting Officer 

1. An anti-lock vehicle brake control system comprising, a source of Fluid pressure to control the braking force on a vehicle wheel, a wheel sensor for producing a train of pulses at a frequency that is related to the vehicle wheel speed, transistor switching means coupled to said wheel sensor to receive said pulse train, a first isolating diode, a first capacitor coupled to said transistor switching means via said first diode, said transistor switching means being responsive to the pulses in said pulse train to cause said first capacitor to be charged to a predetermined voltage level via said first diode, circuit means for discharging said first capacitor during the period between successive pulses in the pulse train, a second capacitor, a charge circuit including a source of voltage permanently connected to said second capacitor, a second isolating diode coupling said first and second capacitors together so that the charge on the second capacitor can decay towards the level of charge on the first capacitor to produce a voltage on the second capacitor that is determined by the wheel speed, and control valve means actuated in response to a given voltage level on said second capacitor to relieve the fluid pressure applied to the vehicle brakes.
 2. A brake control system as claimed in claim 1 wherein said transistor switching means comprises, an input transistor coupled to the wheel sensor, means for biasing said input transistor at its threshold of conduction so that each pulse received turns the transistor on thereby producing at its output electrode a square wave voltage at the pulse frequency, a differentiating circuit coupled to said transistor output electrode for differentiating each cycle of said square wave voltage, a switching transistor which is normally biased into conduction, and means coupling the output electrode of the switching transistor to the first diode and the input of said switching transistor to the output of the differentiating circuit so that the switching transistor is cut-off in response to the leading voltage spike produced therein during each cycle of the square wave voltage.
 3. A brake control system as claimed in claim 2 wherein said switching transistor output electrode is the collector electrode, means connecting said first diode and said first capacitor in series to said collector electrode, a resistor connected in shunt with the first diode, a second resistor connecting said collector to a source of DC voltage, said first capacitor being charged via said second resistor and said first diode when said switching transistor is cut-off.
 4. A brake control system as claimed in claim 3 wherein said circuit means for discharging the first capacitor comprises said shunt resistor and the emitter-collector path of the switching transistor.
 5. A brake control system as claimed in claim 2 wherein said first and second diodes are series connected with opposite polarity between the switching transistor output electrode and the second capacitor.
 6. A brake control system as claimed in claim 1 wherein said transistor switching means comprises, an input transistor coupled to the wheel sensor, means for biasing said input transistor at its threshold of conduction so that the transistor undergoes a change of its conductive state in response to each pulse received, a differentiating circuit coupled to said transistor output electrode for differentiating the output pulses appearing thereat to produce positive going and negative going voltage spikes, the voltage spikes of one polarity being applied through said first diode to charge said first capacitor, and a resistor connected in shunt with said first capacitor to provide a discharge path therefor.
 7. A brake control system as claimed in claim 6 further comprising a third diode connected between the output of the differentiating circuit and ground and poled so as to bypass to ground the voltage spikes of the other polarity. 